The use of silicon-based thin-film photovoltaic devices as photovoltaic devices such as solar cells is already known. These photovoltaic devices generally comprise a first transparent electrode, a silicon-based semiconductor layer (a photovoltaic layer), a second transparent electrode, and a metal electrode film deposited sequentially on top of a substrate. This type of structure, wherein the photovoltaic device contains a single photovoltaic layer, is called a single structure. Furthermore, in order to improve the photovoltaic conversion efficiency of the photovoltaic device, a method is used in which a plurality of photovoltaic layers formed from semiconductors with different band gaps are stacked together. This type of photovoltaic device that uses a plurality of stacked photovoltaic layers is called a multi-junction photovoltaic device, and structures in which two photovoltaic layers with different absorption wavelength bands are stacked are known as tandem structures, whereas structures containing three stacked layers are known as triple structures. Taking a tandem structure photovoltaic device as an example, an amorphous silicon that absorbs short wavelength light is used as the top cell, which refers to the photovoltaic layer on the sunlight incident side of the device, and a microcrystalline silicon that absorbs longer wavelength light is used as the bottom cell, which refers to the other photovoltaic layer, in order to absorb the light not absorbed by the top cell.
The film deposition conditions for this microcrystalline silicon are very different from the film deposition conditions for the amorphous silicon that has conventionally been used for photovoltaic layers. In order to improve the photovoltaic conversion efficiency (the conversion efficiency), improving the film quality of the deposited microcrystalline silicon film is essential.
The above photovoltaic layers formed from thin films of silicon-based semiconductors are deposited using a plasma enhanced CVD method or the like, and in order to reduce the photovoltaic device production costs, accelerating the photovoltaic layer film deposition rate is desirable. For example, patent citation 1 and patent citation 2 disclose conditions for depositing photovoltaic layers formed from crystalline silicon using a plasma enhanced CVD method at a film deposition rate of not less than 1 μm/h (approximately 0.28 nm/s).
Patent Citation 1: Japanese Unexamined Patent Application, Publication No. 2000-174310
Patent Citation 2: Japanese Unexamined Patent Application, Publication No. 2001-237189